Well, Shoppe is essentially a light weight version of the Ecommerce module, which included in the OpenACS. You could theoretically use Shoppe instead of the included Ecommerce module, but it would be more work (porting any Oracle specific stuff) for less functionality (like customer reviews, templating, product catalog, reporting, etc).
You probably would only want to use Shoppe if you didn't want to install the OpenACS. I recommend that you check out the OpenACS ecommerce module instead.
